This invention relates in general to method and apparatus for determining specific volume and more particularly to method and apparatus for precisely determining volume of miniature articles such as gem stones and precious metals.
It is often desirable to determine the exact volume of an irregular shaped object with a very high accuracy. One particular application is for determining the volume of gem stones or precious metal objects such as gold to use in conjunction with a precise weighing of the object to determine the density or specific gravity. The precise determination of the specific gravity can serve as one identification of the particular article and also can serve to aid in determination of the material making up the article. For example, if a yellow metal object has a density of 19.3 g/cm.sup.3, it can be established as gold. Where the article has a slightly different color due to a minor portion of silver or copper, the determination of the specific gravity will indicate the percentage of gold versus the other metal.
One conventional technique for measuring the volume of an irregularly shaped precious metal is to melt the material in a graduated container. This is not only difficult to do from the standpoint of the heat required, but is also destructive of the shape of the article. Another technique used for determining the volume of an irregularly shaped object is to weigh the object in air and weigh the object in water. Since the object in water is buoyed up by a force equal to the amount of displaced liquid, the difference in the two weight measurements divided by the specific gravity of water should give a determination of the volume of the object. This method suffers from the inaccuracies in determining the weight of the object in water. Still another technique for determining specific gravity is to have a multitude of jars of fluids of different known specific gravities. The sample is then placed in different jars seeking to find the fluid in which the sample is most nearly uniformly suspended. This is a tedious process which may not produce an accurate result and the fluids in the jars are easily contaminated resulting in changes in the specific gravity thereof and therefore that of the sample.
U.S. Pat. Nos. 2,296,852 and 3,129,585 disclose devices for determining the volume of substances, typically soil samples utilizing the displacement of a liquid as a measurement of the volume of the substance. In U.S. Pat. No. 2,296,852 the apparatus includes an accurate screw drive mechanism for the reciprocating plunger in a plunger chamber which moves liquid into and out of the sampling chamber. By advancing the plunger to fill the sampling chamber first without a sample and then with a sample the difference in the position of the plunger will record the volume of the sample. In U.S. Pat. No. 3,129,585 pressurization of a piston onto a quantity of liquid before and after insertion of a sample into the liquid results in a difference in the position of a piston rod connected to the piston which then serves as a measure of the volume of the sample. In U.S. Pat. No. 2,667,782 the pressure drop from an inner pressurized chamber which contains the sample into an outer unpressurized chamber is used to indicate the volume of the sample, whereas in U.S. Pat. No. 3,113,448 a change in the volume of a reference system by a fixed amount is compared with the change in volume in a second gaseous system containing the sample to maintain the specific volume of the gas in the second system equal to that of the first system to determine the volume of the sample. In U.S. Pat. No. 3,487,682 a flexible member forming an integral wall of a fluid chamber is moved under pressure to compress a deformable material so that the deformable material is measured in terms of the volume of liquid applied to the flexible member.
It is the object of the present invention to provide a simple and inexpensive yet highly accurate and easy to operate device and method for measuring the volume of samples.
Broadly stated, the present invention, to be described in greater detail below, is directed to apparatus and the method of its use wherein a piston slidably sealed within a container cylinder is provided with an accurately graduated bore sight tube on top of the piston so that fluid displaced from the container cylinder through a bore in the piston into the sight tube indicates the volume of the sample inserted into the cylinder.
One feature and advantage of the present invention is that the sight tube is provided with graduations which are the precise cubic centimeter volume graduations in the sight tube whereby the volume of the sample is then directly read in terms of the same cubic centimeter measurements. This construction avoids the necessity for providing accurate measuring scales correlated to other structures such as the mechanical drive elements of the system. Thus, wear of the moving elements of the system has no affect on the accuracy of the measurement made with the apparatus.
In accordance with another embodiment of the present invention, the assembly containing the sample is vibrated for removal of air bubbles and the lower end of the bore through the piston is tapered to provide a conical opening for guiding the removal of air bubbles which would produce an erroneous determination of the sample volume.
In accordance with still another aspect of the present invention, a basket spring biased up out of the liquid when the piston is removed from the cylinder lifts the sample out of the liquid following a volume measurement to cause drainage of fluid from the sample to help avoid the necessity for repeatedly zeroing the apparatus after every measurement of a sample volume.
In accordance with still another aspect of the present invention a colored fluid having a density between approximately 1.05 and 1.18 g/cm.sup.3 is provided in the container cylinder and having a surface active agent and an anti-vaporization agent for making repeated accurate volume determinations. The surface active agent provides a wetting affect that avoids entrapped or collected air bubbles on the surface of the sample, and the anti-vaporization agent avoids vaporization of the liquid which could result in inaccurate readings where a measurement is taken over an extended period of time.
In accordance with still another aspect of the present invention a colored cross-linked polystyrene sphere is floated on the surface of the fluid and provides a precise highly contrasting point which can be read on the graduated scale for making the volume measurement. In accordance with this aspect of the invention, the sight tube or a backing scale can be provided with a partially mirrored surface so that in sighting a reading the bead is aligned with its reflection for a level sighting. Contamination of the liquid which might affect the surface tension of the fluid and the shape of the meniscus will not affect the accuracy of the reading taken from the bottom of the sphere.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings.